1. Field of the Invention
The present invention is directed to continuous phase frequency shift keying modulation for use in a wireless communication system. In particular, the invention relates to continuous phase frequency shift keying modulation generated during wireless transmissions using a dedicated processor in a closed system. One specific application of the present invention is for wireless communication from an implantable medical device to an external control device in a closed system, wherein the implantable medical device employs a processor dedicated exclusively to generating continuous phase frequency shift keying modulation during uplink wireless communication to the external device while minimizing energy consumption.
2. Description of Related Art
The use of frequency shift keying as a modulation technique is well known and widely used in a large number of different applications. One such application is the use of frequency shift keying modulation during telemetric communications between an external control device and an implantable medical device. For instance, U.S. Pat. No. 4,561,443 discloses a two-way inductive communications link between an external transceiver 32 and an internal transceiver 34 located in a biologically implanted programmable infusion pump (IPIP). Communication from the external transceiver 32 to the IPIP utilizes frequency shift keying at 48.0 kHz and 51.2 kHz while the return communications link from the IPIP utilizes frequency shift keying between AM subcarriers at 1.6 kHz and 3.2 kHz. Both the external and implant sides of the patented device use a physical switch to toggle between the two frequencies. Specifically, external transceiver 32 includes a switch 40 driven by a power driver 44 to toggle between a first crystal oscillator 38 operating a 48.0 kHz and a second crystal oscillator 35 operating at 51.2 kHz. On the implant side, IPIP transceiver 34 also employs a switch 72 for toggling between a first carrier frequency of 1.6 kHz and a second carrier frequency of 3.2 kHz, wherein a timing and control unit 70 is employed to control the switch 72. The use of a physical switch in the design configuration requires a driving signal to toggle between the two carrier frequency states. While generating the FSK modulated signal on either the implant or external side there is no way to ensure perfect synchronization (e.g., zero phase) when switching between the oscillators, thereby resulting in phase discontinuities in the FSK modulated output signal. An illustrative example of a discontinuous phase FSK modulation signal is shown in FIG. 1. A “0” or low bit is represented by a first frequency f0 while a “1” or high bit is represented by a second frequency f1, wherein the first frequency f0 is lower than that of the second frequency f1. The example signal shown in FIG. 1 includes multiple periods, e.g., Tf0, Tf0′, Tf1, Tf1′. As seen in FIG. 1 phase discontinuities are present, e.g., the time period Tf0 and Tf0′ are not equal, while the time period Tf1 and Tf1′ are also not equal. Referring to the specific example depicted in FIG. 1, the phase discontinuity occurs due to the early transition from the low to the high frequency early during the time period Tf0′ (as indicated by the dashed line). As a result of this discontinuity in phase of the FSK modulation signal the wireless transmission is less robust having a negative impact on the demodulation of the recovered signal.
It is therefore desirable to develop circuitry in a closed system employing continuous phase FSK (CPFSK) modulation during wireless communication to improve the robustness and sensitivity of transmissions while minimizing power consumption.